There is a lot of information on buzzing and Furman's, I have always used Panamax with no issues and you can get either the higher end or lower end model never an issue with windings or buzz, there is always the possibility of a bad ground in your house but that can be checked with the $5 plugin checker.
I stand corrected on the transformers.
Good to know you are not alone.
It is funny that Furman is not posting this information on the WWW. I always had a lot of respect for them.
I had a piece of equipment from Z-Systems. It was a digital switcher and had a low level hum/buzz in it. I tried wrapping the transformer in rubber, I isolated the transformer on rubber bushings and nothing helped. I finally remoted the power supply and that took care of the problem. I have had to do that fix on more than one occasion. Sometimes for hum/buzz from the transformer windings sometimes for bad S/N ratios in the unit caused by the proximity of the transformer to the low level audio circuits.
Glad you are getting it solved.
usually it's caused by loose laminations on the transformer, but DC on the AC line can also cause mechanical transformer hum. Try a different circuit or better yet, turn off ALL circuits and unplug everything but the Furman. If it stops, work backwards to find the culprit. It's possible another piece of equipment is causing DC noise on the AC line.